1. Field of the Invention
The present invention relates to a hybrid cable, comprising both electrical conductor wires for power transmission and optical fibres for data transmission. Particularly, the hybrid cable according to the invention is intended for use in a building. More particularly, the hybrid cable according to the present invention is a low voltage (LV) cable.
2. Description of the Prior Art
Raceways are usually provided in a building for power cables installation, said cables distributing power (i.e. electrical energy) to each apartment of the building.
Typically, the power cables run from the power meters—located at the base of the building—to a switchboard present in each apartment of the building.
In the case an optical fibre cable is needed for allowing telecommunication data to be transmitted to an end user living in a given apartment, additional raceways are provided for receiving the optical fibre cables to be installed. Additional raceways are often needed since the size of the raceways already present in the building—and used for housing the power cables—may not allow an optical fibre cable to be installed together with a power cable within the same raceway. Alternatively, raceways of greater sizes are needed to replace the already installed raceways in case the installation of optical fibre cables cannot be performed together with the already installed power cables within the same raceway.
However, drawbacks arise when further additional raceways or raceways of greater size have to be installed for receiving the optical fibre cables. For instance, masonry work is generally required to be made in the building/apartments, thereby remarkably increasing the installation costs. Moreover, in the case masonry work has to be carried out in the building, at least the majority of the building lodgers must agree on the works to be done.
Whenever an optical fibre is included in a cable, the optical fibre requires to be protected from axial and radial stresses, such as those which typically occur onto a cable which is installed within a building raceway. These stresses are due, for example, to the pulling force acting onto the cable during installation thereof, said pulling force giving origin both to axial traction forces and to radial compression forces, whenever the installation path comprises curvilinear portions in combination with rectilinear portions.
The above mentioned stresses are particularly deleterious to the optical fibres as they can cause attenuation phenomena of the transmitted signal, or can even break or seriously damage the optical fibre during installation of the cable.
The optical fibre can be protected by providing the cable with additional protecting elements. However, such a technical solution would be more complex and, moreover, it would cause the cable size to grow to an unacceptable extent, especially in view of the fact that this cable is requested to be installed in a raceway of limited dimensions (and already present in the building structure).
WO 2005/114285 relates to wireline cables having optical fibre(s) used in conjunction with metallic conductors and to the use thereof, in particular for oilfield applications. This document discloses that placing the optical fibres in specific positions and areas of the cable creates a wide variety of means to monitor well bore activity and conditions. For instance, when the optical fibre is placed in a helical position inside the cable (e.g. optical fibres are helically positioned around a central metallic conductor), measurements of downhole physical properties (e.g. temperature or pressure) can be quickly acquired. Conversely, placing the optical fibre in a central position upon the center axis of the cable—a plurality of metallic conductors being helically positioned around the optical fibre—allows for strain measurements. The reciprocal position of electrical conductors and of optical fibre does not vary along the cable.
The cable disclosed by document WO 2005/114285 is not suitable for installation along tortuous and indefinite paths, as may be required when placing the cable in the raceways of a building. In fact, along a bend in the cable path, for instance, the optical fibre of the cable according to document WO 2005/114285 would tend to maintain its cross-sectional position with respect to the conductor wires, thus undergoing relevant axial/radial stresses which would likely cause an unacceptable deterioration of the data transmission property.